Traditional anti-loosening threaded fasteners rely on features that oppose loosening torque force that exist in the fastener's application. Prevailing torque designs employ mechanical means in the threads to increase the amount of torque force required to loosen or remove the fastener, thereby opposing the aforementioned loosening torque force. This mechanical means can be distorted threads, non-metallic coating that covers some of the thread surface, or an additional plastic component, all of which add friction to oppose loosening torque. Prevailing torque fasteners have short re-use lives, with typical service policy being to replace the disassembled fastener with a new one upon reassembly. Another disadvantage of this fastener is that variation in prevailing torque results in added variation in the joint clamp load.
Some designs employ features on the fastener's clamping surface. These features are typically directional in nature and can be characterized as being “teeth” that “bite” or “dig” into the opposing clamped surface, thus raising the required torque force to loosen the fastener. The disadvantage with this fastener is the damage created to the mating surface by disassembly.
Other designs require drilling a hole in the male threaded fastener and using a castellated nut, which has slot across each pair of opposing flats and requires lining up a slot with the hole in the male fastener and installing a cotter pin to prevent loosening. The primary disadvantage of this type of design is cost.
Some designs that eliminate many of the disadvantages of the above described types of fasteners work effectively for those applications that are subjected to vibration or cyclic loading that stretches and relaxes the male fastener, as on an automotive wheel fastening application. However, if use of the clamped joint involves rotation of the assembly around the fastener's axis, such as on an automotive wheel spindle, the loosening torque that is generated by inertia in the starting and stopping of rotation prevents such designs from providing utility.